Various methods for cleaning contaminated soil have been proposed and implemented in the past, starting with excavating the contaminated soil and replacing it with new soil, but overall they are elaborate and require considerable costs to improve the soil.
In this regard, methods using soil conditioners are comparatively simple and also inexpensive, and can reduce the content of harmful substances below the regulatory values by adsorbing the harmful substances by mixing a soil conditioner into the soil.
Conventionally, almost all soil conditioners used for such methods were produced only from adsorbents of harmful substances (cerium or lanthanum hydrates or the like) serving as the main ingredient (see for example, JP 2001-200236A).
Thus, since almost all conventional soil conditioners are produced only from the adsorbents of harmful substances serving as the main ingredient, they have the disadvantage that their soil improving effect is poor considering that relatively expensive adsorbents of harmful substances are used in large amounts.
In particular in soils with a lot of underground water, the above-mentioned disadvantage is clearly noticeable, because harmful substances such as heavy metals are eluted and drained out, so that there is a need for improvement of this aspect.
The present invention addresses this aspect, and it is an object thereof to provide a soil conditioner that can clean and improve soil by effectively adsorbing harmful substances, even if it is soil with a lot of underground water, through efficient use of as little adsorbent of harmful substances as possible.